1. Field
Example embodiments relate to an oxide semiconductor that may be used in a channel of an oxide thin film transistor or applied to a switching device or a driving device of a display apparatus, for example, a flat panel display, a liquid crystal display, or an organic light emitting display device.
2. Description of the Related Art
Thin film transistors are applied in various application fields; in detail, thin film transistors are used as switching or driving devices of display devices. Currently, liquid crystal displays (LCD) are widely used as TV panels, and thus, research is being conducted to develop methods of applying organic light emitting displays to TVs. Display devices for TVs are being developed to meet the market's needs. Market requirements are, for example, large-sized TVs or digital information displays (DID), lower prices, and a higher image quality (e.g., moving image representation, high resolution, contrast ratio, and color representation). In order to meet such demands, a thin film transistor (TFT) having a large-sized substrate, for example, glass, and improved performance, and which may be applied as a switching or driving device of a display device, is required.
An example of the driving or switching device of a display device is an amorphous silicon thin film transistor (a-Si TFT). The a-Si TFT is currently widely used and is uniformly formed on a large substrate of 2 m×2 m or greater at lower costs. However, as the size and image quality of the display device are increased, higher performance is also required of the driving or switching device. Thus an a-Si TFT of the related art having a mobility of 0.5 cm2/Vs may not be sufficient. Accordingly, a high performance TFT having greater mobility than the a-Si TFT and a method of manufacturing the same are required.
A poly-crystal silicon thin film transistor (poly-Si TFT) performing better than the a-Si TFT has a higher mobility of several tens to hundreds cm2/Vs, and thus may be applied to display devices with higher image quality, whereas the higher image quality is not realizable by the a-Si TFT. Also, the poly-Si TFT has less deterioration in terms of the device characteristics. However, in order to manufacture a poly-Si TFT, a more complicated manufacturing process is required than the a-Si TFT, and the manufacturing costs thereof are also increased. Thus, although the poly-Si TFT is appropriate for higher image quality and may be applied to products, e.g., an organic light emitting device (OLED), the poly-Si TFT is not cost competitive. Thus, the application thereof is limited. In addition, the poly-Si TFT has not yet been applied in a manufacturing process using a large-sized substrate of 1 m×1 m or greater due to limited manufacturing equipment or undesirable uniformity. Thus, applying the poly-Si TFT to TV products may be difficult.
Recently, a zinc oxide (ZnO)-based thin film transistor (TFT) has been studied as an oxide semiconductor device. Zinc oxide (ZnO), an In—Zn oxide, and oxides doped with gallium (Ga), magnesium (Mg), aluminum (Al), and/or iron (Fe) may be introduced as ZnO-based materials. A ZnO-based semiconductor device may be manufactured in a lower temperature process, and may be easily manufactured to have a larger surface due to its amorphous nature. Also, a ZnO-based semiconductor film has higher mobility, and thus has improved electrical characteristics like the poly-crystal silicon. Research is currently being conducted to develop use of an oxide-based semiconductor material layer having higher mobility, that is, a ZnO-based material layer, in a channel area of a TFT.